With the recent implementation into practice of a blue light-emitting diode, studies are being aggressively made to develop a white light-emitting diode using the blue diode as the light emission source. The white light-emitting diode is lightweight, uses no mercury and has long life, and therefore demand is expected to rapidly expand in the future. As for the white light-emitting diode, a diode obtained by coating a blue light-emitting element with a mixture of cerium-activated YAG (Y3Al5O12:Ce) powder and epoxy resin is usually employed (see, Japanese Unexamined Patent Publication (Kokai) No. 2000-208815).
However, a highly bright light-emitting device cannot be obtained using a mixture of cerium-activated Y3Al5O12 (YAG:Ce) powder and epoxy resin, because the light is absorbed by the surface defect of the fluorescent powder. Particularly, the above-described effect increases in a layer comprising an epoxy having dispersed therein a fluorescent substance, because complicated reflection and transmission are repeated on the fluorescent powder surface. Also, the refractive index of the fluorescent substance is larger than resin, and therefore the total reflection occurs inside the fluorescent substance every time light passes through the fluorescent substance, and as a result, a large absorption of light is occurs. For these reasons, a highly bright light-emitting diode cannot be obtained for a white light-emitting diode which has a construction in which a fluorescent substance is dispersed in a resin.
Furthermore, the color of YAG:Ce fluorescence is approximately x=0.41 and y=0.56 in the CIE chromaticity coordinates and when mixed with the blue excitation light at 460 nm, since the color tone is controlled on a line connecting the chromatic coordinate of blue light-emitting diode and the chromatic coordinate of YAG:Ce, the color produced is not white, but becomes white mixed with a blue green-color, so that only a white color insufficient in the red component can be realized. In order to solve this problem, another fluorescent powder capable of emitting a red fluorescence is mixed with the YAG:Ce fluorescent powder and this powder mixture is mixed with the resin, thereby adjusting the color tone. In this way, controlling the color tone of a light-emitting diode, a method of mixing a second fluorescent powder, kneading the mixture with a resin, and coating the kneaded product on a light-emitting element to obtain a color tone that was formerly unachievable is being widely employed.
As described above, in conventional light-emitting diodes, a white light-emitting diode that assures a highly bright light after a change in color tone has not been obtained. In addition, there is a problem in that deterioration occurs due to use of an organic material, such as resin at a portion close to the light source.
It could therefore be advantageous to provide a light conversion structure ensuring good light transmission, high brightness and less deterioration, and capable of controlling the light to a desired color tone. It could also be advantageous to provide a light-emitting device, such as light-emitting diode, using the light conversion structure.